Transistor having a vertical channel

ABSTRACT

A resistance variable memory device including a vertical transistor includes an active pillar including a channel region, a source formed in one end of the channel region, and a lightly doped drain (LDD) region and a drain formed in the other end of the channel region, a first gate electrode formed to surround a periphery of the LDD region and having a first work function, and a second gate electrode formed to be connected to the first gate electrode and to surround the channel region and having a second work function that is higher than the first work function.

CROSS-REFERENCES TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.14/634,121 filed on Feb. 27, 2015, which is a division of U.S. patentapplication Ser. No. 13/948,490 filed on Jul. 23, 2013, now abandoned,which claims priority under 35 U.S.C. 119(a) to Korean applicationnumber 10-2013-0021164, filed on Feb. 27, 2013, in the KoreanIntellectual Property Office. The disclosures of each of the foregoingapplications are incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

Exemplary embodiments of the present invention relate to a semiconductorintegrated circuit device, and more particularly, to a transistor, and aresistance variable memory device including the same, and amanufacturing method thereof.

2. Related Art

With the rapid development of mobile and digital informationcommunication and consumer-electronic industry, studies on existingelectronic charge controlled-devices are expected to encounter thelimitation. Thus, new functional memory devices of new concept otherthan the existing electronic charge devices need to be developed. Inparticular, next-generation memory devices with large capacity,ultra-high speed, and ultra-low power are in demand.

Currently, resistive memory devices using a resistance device as amemory medium have been suggested as the next-generation memory devices,and some of the examples are phase-change random access memories(PCRAM5), resistance RAMS (ReRAMs), and magentoresistive RAMs (MRAMs).

The resistive memory devices may be basically configured of a switchingdevice and a resistance device and store data “0” or “1” according to astate of the resistance device.

Even in the resistive memory devices, the first priority is to improvean integration density and to integrate most memory cells in a narrowarea.

To meet these demands, the resistive memory devices have also adoptedthree-dimensional (3D) vertical transistor structures.

However, even in the 3D vertical transistors, thin gate insulatinglayers may be required. Thus, when a high voltage is supplied to a gate,a high electric field is applied to a lightly doped drain (LDD) regionand gate induced drain leakage (GIDL) may be caused.

SUMMARY

According to one aspect of an exemplary embodiment of the presentinvention, a transistor may include an active pillar including a channelregion, a source formed in one end of the channel region, and a lightlydoped drain (LDD) region and a drain formed in the other end of thechannel region, a first gate electrode formed to surround a periphery ofthe LDD region and having a first work function, and a second gateelectrode formed to be connected to the first gate electrode and tosurround the channel region, and having a second work function that ishigher than the first work function.

According to another aspect of an exemplary embodiment of the presentinvention, a resistance variable memory device may include a verticaltransistor including an active pillar including a channel region, asource formed in one end of the channel region, and a lightly dopeddrain (LDD) region and a drain formed in the other end of the channelregion, a first gate electrode formed to surround a periphery of the LDDregion and having a first work function, and a second gate electrodeformed to be connected to the first gate electrode and to surround thechannel region and having a second work function higher than the firstwork function, and a resistive memory structure connected to the drainof the vertical transistor.

According to still another aspect of an exemplary embodiment of thepresent invention, a method of manufacturing a resistance variablesemiconductor device may include forming a source region in asemiconductor substrate, forming a semiconductor layer on the sourceregion, patterning the semiconductor layer to form an active pillar,forming a first gate electrode to surround the active pillar,surrounding an upper region of the first gate electrode with aninsulating layer while exposing a lower region of the first gateelectrode, and forming a second gate electrode by increasing a workfunction of the exposed first gate electrode.

These and other features, aspects, and embodiments of the presentinvention are described below in the section entitled “DETAILEDDESCRIPTION”.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thesubject matter of the present disclosure will be more clearly understoodfrom the following detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view illustrating a resistancevariable memory device including a vertical transistor according to anexemplary implementation of the inventive concept;

FIGS. 2 to 5 are cross-sectional views sequentially illustrating aprocess of manufacturing a vertical transistor of a resistance variablememory device according to an exemplary implementation of the inventiveconcept;

FIG. 6 is a schematic cross-sectional view illustrating a resistancevariable memory device including a vertical transistor according toanother exemplary implementation of the inventive concept;

FIGS. 7 and 8 are cross-sectional views sequentially illustrating aprocess of manufacturing a vertical transistor of FIG. 6;

FIG. 9 is a schematic diagram illustrating a vertical transistoraccording to another exemplary implementation of the inventive concept;and

FIG. 10 is a schematic cross-sectional view illustrating a verticaltransistor according to another exemplary implementation of theinventive concept.

DETAILED DESCRIPTION

Hereinafter, various exemplary embodiments of the present invention willbe described in greater detail with reference to the accompanyingdrawings.

Exemplary embodiments are described herein with reference tocross-sectional illustrations that are schematic illustrations ofexemplary embodiments (and intermediate structures). As such, variationsfrom the shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,exemplary embodiments should not be construed as limited to theparticular shapes of regions illustrated herein but may includedeviations in shapes that result, for example, from manufacturing. Inthe drawings, lengths and sizes of layers and regions may be exaggeratedfor clarity. Like reference numerals in the drawings denote likeelements.

It should be readily understood that the meaning of “on” and “over” inthe present disclosure should be interpreted in the broadest manner suchthat “on” means not only “directly on” but also “on” something with anintermediate feature(s) or a layer(s) therebetween, and that “over”means not only directly on top but also on top of something with anintermediate feature(s) or a layer(s) therebetween. It is also notedthat in this specification, “connected/coupled” refers to one componentnot only directly coupling another component but also indirectlycoupling another component through an intermediate component. Inaddition, a singular form may include a plural form as long as it is notspecifically mentioned in a sentence.

Referring to FIG. 1, a resistance variable memory device 100 accordingto an exemplary embodiment may include a vertical transistor 101 and aresistive memory structure 185.

The vertical transistor 101 may include an active pillar 120, a firstgate electrode 140, and a second gate electrode 160.

A source S may be provided below the active pillar 120 and a drain D isprovided on the active pillar 120. The active pillar 120 between thesource S and the drain D serves as a channel region of the verticaltransistor 101. At this time, the active pillar 120 may be interpretedas a structure including the source S, or the active pillar 120 may havea structure separately formed on the source S. The source S and theactive pillar 120 may be semiconductor layers. Further, a lightly dopeddrain (LDD) region LDD, which is a low concentration impurity region,may be formed in the active pillar 120 between a portion of the activepillar 120 serving as the channel region and the drain D, and thus ashort channel effect may be alleviated.

The first gate electrode 140 may be formed to surround around an upperportion of the active pillar 120 in which the LDD region LDD is formed.The first gate electrode 140 may partially overlap a portion of thedrain D, but the first gate electrode 140 may be formed substantially ona location of the active pillar 120 corresponding to the LDD region LDD.

The second gate electrode 160 may be connected to the first gateelectrode 140 and surround the channel region of the active pillar 120.For example, the second gate electrode 160 may be in contact with thefirst gate electrode 140 and located below the first gate electrode 140.At this time, the first gate electrode 140 may include a material havinga work function lower than that of the second gate electrode 160. Thatis, when the work function of the first gate electrode 140 overlappingthe LDD region LDD is lowered, high electric field characteristicscausing GIDL may be alleviated, and thus the GIDL characteristics of theLDD region LDD and the drain D adjacent to the LDD region LDD may beimproved.

At this time, a gate insulating layer 135 may be interposed between thefirst and second gate electrodes 140 and 160 and the active pillar 120.Various insulating layers such a metal oxide layer and a silicon oxidelayer may be used as the gate insulating layer 135.

The resistive memory structure 185 may be configured of a lowerelectrode 170 and a resistive memory layer 180. The lower electrode 170may be a conductive layer formed on the drain D and provide a currentand a voltage to the resistive memory layer 180. Although notillustrated in FIG. 1, an ohmic layer may be interposed between thelower electrode 170 and the drain D depending on material properties ofthe lower electrode 170. The resistive memory layer 180 may be a layerof which a resistance value is changed according to the voltage andcurrent provided from the lower electrode 170. As the resistive memorylayer 180, a PCMO layer that is a material for a Re RAM, a chalcogenidelayer that is a material for a PCRAM, a magnetic layer that is amaterial for a MRAM, a magnetization reversal device layer that is amaterial for a spin-transfer torque magnetoresistive RAM (STTMRAM), apolymer layer that is a material for a polymer RAM (PoRAM), or the like,may be variously used.

In the vertical transistor according to the exemplary embodiment, thegate electrode is formed of a material having a relatively lower workfunction in the LDD region LDD, which has a lower GIDL barrier and ahigh electric field is applied to, than in the channel region.

As described above, the gate electrode having a relatively low workfunction is disposed around the LDD region LDD to compensate the lowGIDL barrier according to application of the high electric field, andthus leakage current may be reduced.

A method of manufacturing a resistance variable memory device includinga vertical transistor will be described in detail with reference toFIGS. 2 to 5.

Referring to FIG. 2, a source 110 is formed in a semiconductor substrate105 by implementing impurities into an upper portion of thesemiconductor substrate 105. A semiconductor layer is formed on thesemiconductor substrate 105 in which the source 110 is formed. Forexample, the semiconductor layer may be an impurity-doped polysiliconlayer or a layer that epitaxially grows the semiconductor substrate 105in which the source is formed. A hard mask layer 130, for example, asilicon nitride layer is deposited on the semiconductor layer.Predetermined portions of the hard mask layer 130 and the semiconductorlayer are patterned to form a plurality of active pillars 120. A gateinsulating layer 135 is formed on surfaces of the plurality of activepillars 120 and the semiconductor substrate 105. As the gate insulatinglayer 135, a layer in which a conductive material such as silicon (Si),tantalum (Ta), titanium (Ti), barium titanium (Bari), barium zirconium(BaZr), zirconium (Zr), hafnium (Hf), lanthanum (La), aluminum (Al),yttrium (Y), or zirconium suicide (ZrSi) is oxidized, may be used. Afirst conductive layer is deposited on the semiconductor substrate 105including the gate insulating layer 135, and anisotropically etched tosurround the active pillar 120. Therefore, a first gate electrode 140 isformed over an outer circumference of each of the active pillar 120covered with the gate insulating layer 135. At this time, by anisotropicover-etching, the first gate electrode 140 may be formed to have aheight lower than that of the active pillar 120. For example, as thefirst gate electrode 140, a transition metal layer including a metalsuch as Ti, Ta, cobalt (co), or platinum (Pt) may be used.

As illustrated in FIG. 3, a first insulating layer 145 is formed to filla space between the active pillars 120. Next, the first insulating layer145 is recessed to expose an upper region of the first gate electrode140. At this time, an upper surface of the first insulating layer 145may be located to correspond to a channel formation region of the activepillar 120. A second insulating layer 150 is formed to cover the exposedupper region of the first gate electrode 140. The second insulatinglayer 150 may be formed of a material having an etch selectivitydifferent from that of the first insulating layer 145.

Referring to FIG. 4, the first insulating layer 145 is selectivelyremoved to expose a lower region of the first gate electrode 140. Next,nitrogen ions are implanted into the exposed first gate electrode 140 toform a second gate electrode 160 formed of a metal nitride layer, forexample, a titanium nitride (TiN) layer as illustrated in FIG. 5. Asknown, a refractory metal layer such as a Ti layer has a work functionlower than that of a metal nitride layer such as a TiN layer. Therefore,a portion of the gate electrode corresponding to an LDD region LDD isformed of a material having a relatively low work function, and thusleakage current due to GIDL may be reduced.

Next, referring back to FIG. 1, the hard mask layer 130 on the activepillar 120 is removed, and the LDD region LDD is formed by implantingimpurities having a law concentration into the active pillar 120.Subsequently, impurities having a high concentration are implanted inthe active pillar 120 in which the LDD region LDD is formed to define adrain D.

A lower electrode 170 and a resistive memory layer 180 are sequentiallyformed on the drain D to fabricate the resistance variable memorydevice.

A metal silicide layer other than the the a nitride layer may be used asthe second gate electrode 160.

That is, as illustrated in FIG. 6, a first gate electrode 140surrounding the LDD region LDD may be formed of a transition metal layerlike examples in the above-described exemplary embodiment, and a secondgate electrode 165 may be formed of a transition metal silicide layerlocated below the first gate electrode 140 and having a work functionhigher than that of the first gate electrode 140. At this time, athickness b of the second gate electrode 165 may be larger than athickness a of the first gate electrode 140.

Since the transition metal layer also has a work function lower thanthat of the transition metal silicide layer, leakage current around theLDD region LDD having weak GIDL characteristic may be reduced.

A method of manufacturing the vertical transistor illustrated in FIG. 6will be described in detail with reference to FIGS. 7 and 8. Here, someof the manufacturing method of the resistance variable memory device inthe exemplary embodiment are substantially the same as the processes ofFIGS. 1 to 3 in the manufacturing method of the resistance variablememory device in the above-described exemplary embodiment, and thusprocesses subsequent to the process of FIG. 3 will be described.

Referring to FIG. 7, the first insulating layer (145 of FIG. 3) isselectively removed to expose a sidewall of the first gate electrode140. A silicon layer 163 is deposited on an exposed surface of the firstgate electrode 140 to a predetermined thickness. The silicon layer 163may be formed to be located below a second insulating layer 150.

Referring to FIG. 8, a heat treatment is performed on the semiconductorsubstrate 105 so that the first gate electrode 140 is reacted with thesilicon layer 163 in contact with the first gate electrode 140 to formthe second gate electrode 165 formed of a transition metal silicidelayer. At this time, since the second gate electrode 165 is a layerformed through the heat reaction of the first gate electrode 140 withthe silicon layer 163, a thickness of the silicon layer 163 may beprovided as a thickness of the second gate electrode 165. Therefore, thesecond gate electrode 165 may have a thickness larger than that of thefirst gate electrode 140.

As illustrated in FIG. 9, first and second gate electrodes 142 and 167may be sequentially formed to surround an active pillar 120.

That is, the first gate electrode 142 is formed to surround an outercircumference of the active pillar 120 covered with the gate insulatinglayer 135. At this time, it is important that the first gate electrode142 is formed not to overlap an LDD region LDD.

Next, the second gate electrode 167 is formed to surround an outercircumference of the gate electrode 142. At this time, the second gateelectrode 167 may extend by a predetermined length c more than the firstgate electrode 142 so that the second gate electrode 167 overlap aportion of the LDD region LDD. Therefore, for example, only a portion ofthe insulating layer 135 is present between the LDD region LDD and thesecond gate electrode 167 without interposing of the first gateelectrode 142. Here, the second gate electrode 167 may have a workfunction higher than that of the first gate electrode 142. However, insome cases, the second gate electrode 167 may be formed of a materialhaving a work function similar to or lower than that of the first gateelectrode 142.

In the vertical transistor having the above-described structure, sincethe first gate electrode 142 is formed to have a relatively low workfunction and a distance between the LDD region LDD and the second gateelectrode 167 overlapping the LDD region LDD is increased, a highelectric field applied to the LDD region LDD may be alleviated andleakage current due to low GIDL may be reduced.

In addition to the vertical transistor structure, the dual gateelectrode structure may be applied to a buried gate electrode structure.

That is, as illustrated in FIG. 10, a trench 210 is formed in asemiconductor substrate 200. A source S and a drain D are formed in thesemiconductor substrate 200 at both sides of the trench 210.

A first electrode 220 and a second electrode 230 may be formed in thetrench 210 in which a gate insulating layer 215 is formed. The firstgate electrode 220 may be formed on an inner surface of the trench 210.The first gate electrode 220 may be formed to be located substantiallyin a lower portion of the trench 210 so that the first gate electrode220 may not overlap the source S and the drain D.

The second gate electrode 230 may be formed to fill the inside of thetrench 210 surrounded with the first gate electrode 220. At this time,the second gate electrode 230 may be formed to have a height longer thanthat of the first gate electrode 220 so that the second gate electrode230 may overlap portions of the source S and the drain D.

Although not shown in FIG. 10, it would have been obvious to a personhaving ordinary skill in the art that the resistive memory structure 185illustrated in FIGS. 2 and 6 may be additionally formed. The referencenumerals 240 and 250 denote insulating layers.

Therefore, a region around the drain D overlaps the second gateelectrode 230 without interposing of the first gate electrode 220.Accordingly, a distance of the region around the drain affected by thehigh electric field to the gate electrode is substantially increased sothat the GIDL effect may be reduced.

Further, since the first gate electrode 220 is formed of a materialhaving a work function lower than that of the second gate electrode 230,an effect of the electric field on the region around the drain D, thatis, a region corresponding to the LDD region LDD may be furtheralleviated.

Further, the second gate electrode 230 may be formed to fill the insideof the trench 210 surrounded with the first gate electrode 220.

As specifically described above, according to the exemplary embodiments,since the gate electrode having a relatively low work function is formedaround the LDD region LDD, the low GIDL barrier due to application of ahigh electric field may be compensated and the leakage current may bereduced.

The above embodiments of the present invention are illustrative, and theinvention is not limited by the embodiments described above. Variousalternatives and equivalents are possible, and the invention is notlimited to any specific type of semiconductor device. Other additions,subtractions, or modifications may be made in view of the presentdisclosure and are intended to fall within the scope of the followingclaims.

What is claimed is:
 1. A semiconductor device, comprising: an activepillar extending from a substrate in a first direction and including asource and a drain, wherein the source and the drain are arranged in thefirst direction; a channel region formed in the active pillar andbetween the source and the drain; a lightly doped drain (LDD) regionformed in the active pillar and between the drain and the channelregion; a gate insulating layer formed over a sidewall of the activepillar in a second direction, wherein the second direction isperpendicular to the first direction; a first gate electrode formed overthe gate insulating layer in the second direction, entirely surroundingthe source and the channel region, and having a first work function; anda second gate electrode formed over the first gate electrode in thesecond direction and having a second work function that is higher thanthe first work function, wherein the first gate electrode is formed overan outer circumference of the active pillar, and the second gateelectrode is formed over an outer circumference of the first gateelectrode, wherein a total height of the first gate electrode is shorterthan a total height of the second gate electrode, wherein each of thetotal height of the first gate electrode and the total height of thesecond gate electrode is measured only in the first direction, andwherein the first gate electrode is positioned between the gateinsulating layer, which is formed over the sidewall of the activepillar, and the second gate electrode only in the second direction. 2.The semiconductor device of claim 1, further comprising a resistivememory structure connected to the drain.
 3. The semiconductor device ofclaim 2, wherein the resistive memory structure includes: a lowerelectrode formed over the drain in the first direction; and a resistivememory layer formed over the lower electrode in the first direction. 4.The semiconductor device of claim 3, wherein the resistive memory layerincludes one selected from the group comprising a PrCaMnO₃ (PCMO) layerincluding a material for a resistance random access memory (ReRAM), achalcogenide layer including a material for a phase-change RAM (PCRAM),a magnetic layer including a material for a magnetoresistive RAM (MRAM),a magnetization reversal device layer including a material for aspin-transfer torque magnetoresistive RAM (STTMRAM), and a polymer layerincluding a material for a polymer RAM (PoRAM).